As set forth below, the power spring of a pneumatic spring brake actuator serves as the emergency brake in the event of failure of the pneumatic braking system of heavy duty vehicles, including trucks, trailers, buses, motor coaches and the like. The power spring also serves as the parking brake for such vehicles. However, there is no industry recognized method of checking the condition of the power spring of a spring brake actuator other than to disassemble the brake actuator. Further, most spring brake actuators are now tamper resistant, such as disclosed in U.S. Pat. No. 4,960,036 assigned to the Assignee of this application, to avoid inadvertent release of the power spring resulting in death or injury. In a tamper resistant spring brake actuator, the power spring chamber is permanently sealed, such that the spring chamber cannot be opened to visually inspect the power spring. Second, to determine the condition of the power spring of a spring brake actuator, it is necessary to differentiate a decrease, for example, in the push rod stroke resulting from other conditions of the vehicle braking system, including a worn or out of adjustment slack adjuster or foundation brake and other potential failures of the spring brake actuator. Thus, it is not possible to simply measure the stroke of the push rod of a spring brake actuator to determine condition of the power spring.
There is therefore a need for a method to monitor the condition of a power spring of a pneumatic spring brake actuator.
A conventional “piggyback” pneumatic spring brake actuator includes a housing which is divided into a power spring chamber or “spring chamber” and a service chamber, also referred to as the “non-pressure” housing. A power spring is normally compressed in the power spring chamber by the pneumatic pressure of the braking system of the vehicle. In a “dual diaphragm” spring brake actuator, for example, the spring chamber includes a flexible cup-shaped diaphragm and pneumatic pressure from the vehicle reacts against the diaphragm to normally compress the power spring when the vehicle is operating. The power spring chamber further includes a piston having one end biased against the diaphragm and a second end which reciprocates into the service chamber upon expansion of the power spring. The service chamber of a dual diaphragm spring brake actuator also includes a cup-shaped flexible diaphragm and a push rod which is biased against the diaphragm in the service chamber, such that upon normal actuation of the braking system of the vehicle, pneumatic pressure received in the service chamber inverts the diaphragm in the service chamber, which drives the push rod through an opening in the housing to actuate the foundation brake of the vehicle. In a typical application, the push rod is connected to a slack adjuster which is connected to a linkage connected to the foundation brake of the vehicle.
Thus, the braking system of the vehicle is actuated by pneumatic pressure of the vehicle received in the service chamber. Upon depression of the brake pedal the pneumatic pressure inverts the diaphragm in the service chamber, which extends the push rod to actuate the foundation brake of the vehicle. However, when the pneumatic pressure of the vehicle falls below a predetermined minimum pressure or the parking brake is actuated to release pneumatic pressure from the power spring chamber, the power spring expands, driving the piston in the spring chamber into the service chamber, thereby driving the push rod in the service chamber to actuate the foundation brake of the vehicle. Although the power spring of a pneumatic spring brake actuator is generally not subject to failure, at present there is no industry recognized method for periodically checking the condition of the power spring of a spring brake actuator as set forth above. The method of this invention permits monitoring the condition of the power spring of a pneumatic spring brake actuator without disassembling the power spring chamber, permitting periodic checking of the condition of the power spring.
The prior art also discloses various methods and apparatus for monitoring the stroke of the push rod of a spring brake actuator to determine whether the spring brake actuator or the foundation braking system of the vehicle require maintenance. U.S. Pat. No. 6,255,941 assigned to the Assignee of this application discloses a commercial brake monitoring system of the assignee of this application which provides continuous or periodic monitoring of the stroke of the push rod of a brake actuator. However, as set forth above, the prior art does not disclose a method of monitoring the condition of the power spring using this technology. The method of the present invention permits monitoring the condition of the power spring of a spring brake actuator by monitoring the stroke of the push rod and a defective power spring may be signaled to the operator or maintenance person.